We have found that colchicine inhibits the secretion of plasma proteins by rat liver without affecting protein synthesis or the energy supply levels of the cell. Secretion involves the transcellular movement of newly formed proteins from membrane-attached polysomes to the endoplasmic reticulum and to the Golgi where proteins are finally assembled and packaged for export. We shall determine at what stage of this process colchicine acts, and whether it acts by directly interacting with cytoplasmic microtubules or by some other effect.